Validation-SourceType lost due to inheritance?

my GUI is based on standard forms. The ValidationProvider is part of the baseclass-form (protected). In the base form it is connected to an errorprovider and enabled, but no SourceType is being set. The derived form uses databinding and validation. Thus I set
the SourceTypeName and the SourceProperties for the controls (mainly textboxes) in the inherited form. During input of data there is nevertheless a NullReferenceException, because SourceType is null. Stepping through the initialization of the form, the SourceType
is correctly set, I can't find the point, there the SourceType is lost. Any suggestions?